1. Field of the Invention
This invention relates to an electrophotographic photoconductor for use in image forming machines such as printers, copying machines and facsimile machines. The present invention is also directed to a method of preparing an electrophotographic photoconductor.
2. Description of the Related Art
Recent development of information processing systems using electrophotography is striking. In particular, laser printers and digital copying machines in which information is recorded using light as digital signals have shown significant improvement in image quality and reliability. Further, such an improvement is now coupled with high speed recording technique to provide full color laser printers and full color digital copying machines. In this circumstance, a need exists for an electrophotographic photoconductor which can provide high quality images and which has high durability.
As to an electrophotographic photoconductor for use in laser printers and digital copying machines, the recent trend is toward the use of organic photoconductive materials for reasons of their low costs, good productivity and freedom of pollution. Typical examples of known organic electrophotographic photoconductors include those using a photoconductive resin such as polyvinylcarbazole (PVK), those using a charge transport complex such as 2,4,7-trinitrofluorenone (PVK-TNF), those using a dispersed pigment such as phthalocyanine-binder, and those of a function-separation type using a combination of a charge generation material and a charge transport material. Above all, the function-separation type electrophotographic photoconductor is currently dominant.
The mechanism of forming electrostatic latent images using the function-separation type electrophotographic photoconductor is as follows. First, a surface of the photoconductor is charged and thereafter exposed to light images. The light passes through the charge transport layer and enters the charge generation layer so that a charge generation material contained therein absorbs the light, whereupon a charge carrier is produced from the charge generation material. The charge carrier is injected into the charge transport layer and travels along an electric field generated by the charging step to neutralize the surface charge of the photoconductor. As a result, electrostatic latent images are formed on the surface of the photoconductor.
In general, the organic electrophotographic photoconductors have drawbacks because the surface thereof is apt to be abraded upon repeated use, which causes a reduction of the charge potential and the light sensitivity thereof as well as injuries of the surface thereof, resulting in deterioration of the image quality such as an increase of background stains and a reduction of image density. For this reason, it is one of the important problems to provide an electrophotographic photoconductor having high durability. Recent demand for high speed and compact image forming apparatuses also calls for high durability of the photoconductor. The organic electrophotographic photoconductors have additional drawbacks because cracks are apt to form when foreign matters such as lipids derived from human hands deposit on a surface thereof and when crystallization of the photoconductive material occurs on the deposited area.
To cope with these problems, a method has been proposed in which a protective layer is provided on the top surface layer of the photoconductor. The protective layer may be lubricated, cured, or added with a filler. For example, Japanese Laid-Open Patent Publications No. 07-295248, No. 07-301936 and No. 08-082940 disclose incorporating a fluorine-modified silicone oil into the outermost surface layer of a photoconductor for the purpose of improving surface characteristics such as wear resistance thereof. However, the fluorine-modified silicone oil tends to be lost during repeated use and fails to maintain its effect for a long time.
A proposal has been made to incorporate an inorganic filler or crosslinked resin particles in the outermost layer of an organic electrophotographic photoconductor to improve the abrasion resistance. The use of the filler, however, adversely affects the charging potential and residual potential and causes a problem that the potential greatly varies upon repeated use of the photoconductor.
Also proposed is the use of a thermosetting resin in the outermost layer. In particular, much attention is being paid on a three-dimensionally crosslinked resin containing a charge transferability-imparting skeleton in view of its good electrophotographic properties and good wear resistance. For example, Japanese Laid-Open Patent Publication No. H09-190004 discloses the use of organic silicon-modified positive hole transporting compound in which a silicon compound having a hydrolyzable group is directly introduced into an charge transporting material. Japanese Laid-Open Patent Publication No. 2000-171990 suggests the use of an outermost layer in which a charge transporting compound having a hydroxyl group, an amino group or a thiol group capable of condensing with a silanol group is introduced into a three-dimensionally crosslinked siloxane layer. Japanese Laid-Open Patent Publication No. 2000-206715 proposes the use of a three-dimensionally crosslinked layer obtained from an acrylic compound and a charge transporting material having at least two chain-polymerizable groups.
The electrophotographic photoconductor of Japanese Laid-Open Patent Publication No. H09-190004, however, has a problem that it is difficult to synthesize and purify the organic silicon-modified positive hole transporting compound. An unpurified product causes a reduction in layer strength and adversely affects the electrophotographic properties. The photoconductor of Japanese Laid-Open Patent Publication No. 2000-171990 has drawbacks that the charge transporting compound having incorporated thereinto a reactive group has poor compatibility and the mechanical strength of a layer obtained therefrom are not satisfactory. In the case of the photoconductor of Japanese Laid-Open Patent Publication No. 2000-206715, the residual chain-polymerizable groups adversely affect the resistance to gases and the mechanical strengths of a layer obtained therefrom.